1. Field of the Invention
This invention relates to a process for manufacturing substitute pipeline gas by methanation, the product gas having a calorific value of about 1000 Btu per standard cubic foot, from water gas rich in carbon monoxide and hydrogen. Typical of such water gas is the raw gas generated by known and conventional coal gasification processes.
2. Description of the Prior Art
To convert gas rich in carbon monoxide and hydrogen to substitute pipeline gas, it is generally considered necessary to produce it by the following steps: desulfurization; shift conversion to produce a gas with H.sub.2 /CO ratio of about 3:1; CO.sub.2 extraction; methanation; and dehydration.
A number of processes have been proposed for the manufacture of substitute pipeline gas from coal. Typical of these processes are the HYGAS Process proposed by the Institute of Gas Technology, the CSC Process of Consolidation Coal Co., the Bi-Gas Process of Bituminous Coal Research, Inc., the Synthane Process of the Bureau of Mines, and the COGAS Process of FMC Corporation. An older process which has been employed commercially for the manufacture of low Btu gas is the Lurgi Process of the Lurgi Gesellschaft of Frankfurt, Germany. All of these processes generate raw gases which are quite rich in carbon monoxide and hydrogen, but are generally free of nitrogen or argon. By combining carbon monoxide and hydrogen via the methanation reaction over a suitable catalyst, the raw gases can be converted to high Btu gas comprising predominantly methane.
The methanation reaction is EQU CO + 3H.sub.2 = CH.sub.4 + H.sub.2 O
it has generally been considered necessary to pre-adjust the H.sub.2 /CO ratio to between 3.0:1 and 3.1:1. Since the H.sub.2 /CO ratio of the raw gas is substantially below this range, the carbon monoxide in the gas is first reacted with steam over an iron catalyst via the "shift" reaction EQU CO + H.sub.2 O = CO.sub.2 + H.sub.2
this reaction does not normally go to completion. The degree of completeness is limited by equilibrium, which is in turn dependent on the temperature and the concentration of the active species (CO, H.sub.2 O, CO.sub.2, and H.sub.2). By an appropriate choice of process conditions it is possible to arrive at a composition wherein the ratio of H.sub.2 to CO is in the desired range. Alternatively, a part of the gas can be reacted to a H.sub.2 /CO ratio substantially in excess of 3:1, and the reacted gas then mixed with the unreacted remainder to produce the desired 3 to 1 ratio.
At this point, the gas will have a temperature of about 700.degree. F or higher and will generally contain excess CO.sub.2 and steam; it may also contain deleterious impurities such as sulfur compounds. The latter must be removed to residuals substantially under 5 ppm and preferably less than 0.2 ppm to protect the nickel catalysts used for methanation which are poisoned by sulfur.
To remove CO.sub.2 and sulfur compounds, the gas is cooled to 100.degree.-200.degree. F and treated with alkaline absorbents such as the ethanolamines or the promoted alkali carbonates. If the sulfur compounds are present in quantities higher than about 50-100 ppm, some selective process for extraction of sulfur is preferable so as to make possible the recovery of sulfur in a stream of sufficient concentration to permit conversion to sulfur or sulfuric acid in a practically realizable process. If less than 50 ppm, it is usually more economical to absorb the sulfur on zinc oxide or similar absorbents which can be disposed of by batch removal from the system. A combination of sulfur extraction and final cleanup by absorption on zinc oxide may also be used. This part of the technology is well known and the selection of the specific route is governed mainly by economics.
The gas comprising some pre-formed methane, but now largely hydrogen and carbon monoxide, is ready for methanation. This reaction, EQU CO + 3H.sub.2 = CH.sub.4 + H.sub.2 O
takes place over nickel catalyst and is very highly exothermic. In order for the reaction to start, the gas generally has to be at a temperature of 400.degree. F to 450.degree. F, and therefore, it has to be reheated. Generally, this is done by exchange against the hot effluent from the converter.
Exothermic reaction heat is evolved in such large quantities that if uncontrolled, temperatures would rise to levels which could damage the catalyst and even the reactor vessel. To control the temperature rise, a number of process schemes have been proposed. These schemes generally involve the recycling of methanated product, as well as the staging of the reaction in a series of discrete steps with intercooling between stages.
In one such scheme, raw gas having a H.sub.2 /CO ratio equal to about 1.6:1 is cooled, scrubbed, and the particulate matter removed therefrom in step (1), desulfurized in step (2), reheated in step (3), subjected to shift conversion (CO+H.sub.2 O=CO.sub.2 +H.sub.2) in the presence of steam in step (4), cooled and the CO.sub.2 removed therefrom in step (5), reheated in step (6), subjected to methanation in step (7) with by-product steam recycled to process, cooled and compressed in step (8) with some product methane being recycled to the methanation reactor to moderate the reaction and dehydrated in step (9).
An example of such a known process is generally described in U.S. Pat. No. 3,511,624, which discloses the catalytic methanation, in at least 2 stages, of mixtures containing carbon monoxide, hydrogen, steam and at least 25% by volume of methane. The mixture is passed in a first stage over a methanation catalyst which is at a temperature of from 200.degree. C to 450.degree. C; steam is then partially removed from the mixture which is then passed into a second stage over a methanation catalyst which is at a temperature within a range lower than the temperature of the mixture leaving the first stage; steam and carbon dioxide are subsequently removed from the mixture. The amount of steam present in each stage is at least sufficient to prevent carbon deposition on the catalyst.
U.S. Pat. No. 3,515,527 discloses a catalyst that can be used in the invention comprising nickel, alumina and an oxide, hydroxide or carbonate of an alkaline earth metal such as barium, the nickel and alumina having been prepared by co-precipitation, the portion being such as to provide from 10% to 30% by weight of the alkaline earth metal and from 25% to 75% by weight of the nickel based on the combined weight of the nickel, alumina and alkaline earth metal. This patent further discloses the production of gases containing methane by reaction of a predominantly hydrocarbon feedstock having a final boiling point of not more than 300.degree. F with steam by passing the feedstock in vapor form and steam at a temperature of from 300.degree. C to 600.degree. C into a bed of the catalyst whereby the bed is maintained at temperatures within the range of 500.degree. C to 600.degree. C.